DISTRIBUTION OF RADIONUCLIDES IN POLLUTED SOILS, LAKE AND
OIL-FIELD WATERS
A .A .G a rib o v , H .M .M a h m u d o v , F .R M u ra d o v , G .Z .V elib ey o v Institute of Radiation Problems ANAS, AZ1143, Baku, F.Agayev str.9 E-mail: hokman@rambler.ru
1. INTRODUCTION
It is for many years that Institute of Radiation Problems ANAS has started works on fundamental research of radiation resistance of territories of oil-and-gas production enterprises and its radionuclide composition. A core laboratories were created, provided by modem measuring devices and strong group of specialists was trained in order to fulfil these works and manage the obtained results perfectly. The researches were conducted in the territory of OGPE.
The analysis of the samples taken from different oil and gas fields shows that U238 and Th232 isotopes are not observed in composition of solid substances extracted from the wells and the surface of oil-and-gas production equipment. Thus these elements penetrate oil, gas and oil-field water from oil strata of the fields. While the presence of II group elements in process water, decrease of pressure and temperature may make them exceed solubility limit of mixed sulphates and carbonates. And this cause them accumulate in internal walls of pipes, valves, pumps and separators as sulphate and carbonate sediments. As the presence of turbulent current, centripetal forces and crystallization centers makes a favorable condition for crystals nucleation, sediments generate. Clay and sand particles extracted from productive layer may be the reasons of superficial sedimentation. If oil-field water was mixed with sea water in order to increase oil extraction, then concentration of sulphates increased and the process of sedimentation strengthened. Such mixture may also occur during penetration of sea water into layer and this leads to generation of sediments in wells. Besides this, the probability of mixture of oil-field waters extracted from different wells in surface equipments and generation of sediments is very high [1, 4,5].
Oil-field waters extracted together with oil are spilled by channels and collected in artificial lakes when radionuclides extracted from layers accumulate around channels and lake waters. At the result of sedimentation and accumulation processes a large amount of Ra and Ra isotopes and their decomposition products are observed in soil, water and bottom sediments. And this leads to formation of high y-radiation background in oil-produsing areas. Solar plant and pipes’ bloom used during oil-extraction process are strong radiation sources in most cases [3, 5].
High dose rate generates around components of equipment contaminated by internal sediments and slimes polluted by radionuclides and it depends on amount and spesific activity of sediments, protective attributes of pipes and capacity walls. Maximal values of dose rate measured around the equipment are usually several mkR/hours, but in some cases may reach lOOmkR/hours and more which exceed the values of natural radiation background for many times [1,2].
2. TECHNIQUES OF THE RESEARCH
Soil and water samples were taken from the lakes in the territory, the wells in different layers in order to investigate radionuclide composition of the oil-polluted areas. While taking samples pH-value of environment is reached to 2 by adding chloride into it. 1 kg soil sample and 1-10 litre water sample are taken.
Water sample was prepared for analysis in the following way:
1. The sample needed for analysis (1-10 1) is filtrated by paper filter; 2. 1 1 cylindrical mount is filled by sorbent of cation exchange;
3. The sample is filtrated by this mount and all cations, as well as radio-nuclides remain in sorbent. 4. The sorbent is dried up in drier at the temperature 50°C for 24 hours;
7. The weight of the vessel is defined with 1 gram accuracy together with sorbent;
8. According to difference of the weights of full and empty vessels the weight of the sorbent in them is calculated;
9. The sorbent is kept in hermetically sealed Marinelly vessel for a month in order to reach radio-nuclides in them to radioactive equilibrium state.
10. After it is kept for a month, the sorbent is analyzed in y-spectrometer device.
Analysis o f the sample in “Progress-gamma ”spectrometer. “Progress-gamma” (Dose, Russia) y-
spectrometer was used to analyze radio-nuclide composition of the samples. The detector of this spectrometer is scintillation detector developed on the base of NaL (TL) crystal. Detection range of the device is 200 - 2800 keV. The device is controlled by computer using special “Progress” program. Energy calibration of scintillation y-spectrometric path is conducted automatically using peaks of 137Cs and 40K radio-nuclides. For this purpose two-component 137Cs+40K calibration source included into unit of equipment is used and this value should not differ from the one shown in the certificate of the device for 10%. After this empty Marinelly vessel is put into the device and background spectrum is measured during an hour. The obtained spectrum is automatically processed and specific activity of the radio-nuclides observed is calculated by the program. Scintillation spectra are processed by matrix method in “Progress” program. By this way the activity of the radio-nuclides covered each other by their peaks is defined with more accuracy in comparison with primitive traditional methods. Specific activity of radio-nuclides for water samples is calculated using the result obtained for the sorbent by the following formula:
A
waterA
sorbentmsorbentV
waterburada Awater - specific activity of radio-nuclide in water, Asorbent - specific activity of radio-nuclide in sorbent, msorbent - weight of sorbent in Marinelly vessel, Vwater- volume of water sample.
3. THE OBTAINED RESULTS
Exposition dose rate of y-rays created by radio-nuclides accumulated in local areas in oil-and-gas production and its preliminary treatment for years was determined and radiation bachground of most areas varies in the range of 4-8 mkR/hours.
The analysis of the samples taken from local araeas with hiugh radiation background shows that they are mainly natural radio-nuclides 40K, 232Th ba 226Ra. These radio-nuclides may be observed maonly in mucks extracted from the layers of different depths in oil-and gas production process which accumulate in different areas when they are extracted from together with oil-field waters. On Figure 1 distribution dependence of natural radio-nuclides according to depth in soils polluted by them in oil-production areas was shown. The researches conducted show that distribution of natural radio-nuclides depends on a certain law and decreases exponentially on the surface of polluted areas and in several depths. It was revealed that natural radio nuclides mainly accumulate on the surface od soil, the reason must be rise of oil-field waters and weak migration of natural radionuclides into depth [1].
As it seems from the graphic that Ra isotopes are observed only on very thin surface layer in soil the surface of which was polluted by natural radio-nuclides. When depth increases, their activity decreases sharply. And this maintains that Ra in soil is in the form of compounds with weak migration.
Figure 1. Distribution of natural radio-nuclide isotopes according to depth
Radionuclides are mainly observed in mucks extracted from layers of different depths during oil-and gas production process which accumulate in several areas while extracting together with oil-field waters. On Figure 2 the results of the radionuclides’ analysis according to depth were shown. As it seems that absolute value of radionuclides’ volumetric activity independent on depth varies chaotically. Radionuclides’ concentration in the taken water samples according to depth varies in the range of " 6Ra-3.14, " sRa-4.1, l1jK- 3.14 Bq/L in 950-1070 meters, but 226Ra-2.7, 228Ra-2.3, 40K-4.21 Bk/L in 4000-4200 meters which doesn’t depend on any law. Despite of all of them in big time period radionuclides accumulate in local areas and create a big natural radiation background [1,2,4,5].
According to the conducted researches volumetric activity of the radionuclides solved in oil-field waters of different depths is equal to 5-10% error. Graphic of radionuclides’ distribtion according to depth was given on Figure 1. ^2 pq vs CD 3O G G o 'o 6 5 4 3 2 1 0 o 2 2 6 R a □ 228Ra A40K P a C__________ * CP*P A A □ A A A M A O A B D O O O D D 3 A O □ 8 8 SS8 0 1000 2000 3000 4000 500C Layer depth, m
Figure 2. Distribution of radio-nuclides in water samples taken from the layers of different depths in the territory of OGPE
Table 1 shows that the amount of radio-nuclides in lake waters is small, at sensitivity limit of methods of analysis and lower than that. But volumetric activity of radio-nuclides in oil-field waters spilling through channels into these lakes is high for many times. K40 isotope is detected in oil-field water as well as lake water samples taken from the channel. It is explained by the fact that as potassium is an alkali metal, most of its inorganic compounds solve in water well and is spilled into lake by channel in solved form. There they always remain in water without forming sediments. As activity of 40K isotope in lake water is lower than that in oil-field water, it is explained by the fact that oil-field water mix with rain water and other waste waters which don’t contain radioactive elements.
Table 1. Radio-nuclide composition of the samples taken from lakes and channels
JVo Sample
Volumetric activity, Bk/l
K-40 Ra-226 Ra-228
1
Oil-field water spilling into lake (channel) 3.8 ± 1.0 0.35 ±0.10 0.37 ±0.12 2 Lake water 1.5 ±0.7 <0.21 <0.24 3 Lake water 1.2 ±0.6 <0.21 <0.24 4 Lake water 1.7 ±0.9 0.22 ±0.11 0.26 ±0.12 5 Lake water 1.3 ±0.6 <0.21 <0.24
While comparing activities of 226Ra and 228Ra isotopes in water sample taken from the channel with other four samples great differences are noticed. When oil-field waters flow through the channel radium in its composition forms compounds insoluble in water and gradually sediment during the process of running. So decrease of volumetric activities of Ra and Ra isotopes is observed along the channel. Therefore these radio-nuclides and their decomposition products are encountered in large amounts in composition of the soil around the channels. If the length of the channel is long enough, a large amount of radium in composition of water sediments and remains in a small amount in composition of the water spilled into the lake. On the other hand, as water remains in lake for a long time, residual radium in the composition of water spilling into this lake almost sediment completely. Therefore though radium isotopes are not detected in composition of lake water, in some cases bottom sediments of that lake are detected [2, 5, 8].
It is important to realize some urgent measures in order to ensure radiation safety of oil-and gas production industry which include treatment of gas products, soil and water by effective and productive methods, landfill of solid radioactive wastes, deactivation of equipments and so on.
4. CONCLUSION
1. Migration of radio-nuclides in soil the surface of which was polluted by natural radionuclides according to depth is weak, therefore isotopes accumulate mainly on soil surface. This is explained by the fact that Ra isotopes are in the form of compounds migrating in soil.
2. Investigation of oil-field waters shows that mainly daughter products of natural radionuclides were solved in them, changes in the range of (226Ra (0.46-5.14), 228Ra (0.57-4.12) and 40K (0.36-4.21) Bk/L) , their distribution according to depth is the same.
5. REFERENCES
2. Garibov A.A., Mehdiyeva R.N., Mahmudov H.M., Muradov F.R./ Investigation of “GUM ADASY” OGPE Production Areas and Radioecological State of the Territory / “Protection of Ecology and Life Activity” V International Scientific Conference, November 24-27, 2004, Sumgait, p. 174-177.
3. H.M.Mahmudov, F.M.Muradov. Study of Radio-nuclide Composition of Oil-field Waters ICNRP’07, The Sixth International Conference “Nuclear and Radiation Physics”. Kazakhstan, Alma-Ata-2007, p.506-507
4. Radiation Safety and Safety of Wastes in Oil-gas Industry, Report of IAEA-2000, p.64
5. V.P.Solodukhin, I.V.Kazachevskiy, S.V. Reznikov and others, Measurement of Radioactive Level in Extraction, Treatment and Transportation of Gas-oil Raw Materials, Equipment and News of Radiation Measurements (ENRM)-2000. N°3., p. 10-14.
